This proposal is concerned with the quantitative basis of cell- mediated immune responses and delayed-type hypersensitivity, particularly in enumerating thymus-derived lymphocytes involved. The virus plaque assay has been found to measure activated T-cells selectively and will be used to explore the relationships between the T- cell measured in the virus plaque assay, the effector cell in delayed- type hypersensitivity and the helper cell in antibody formation. The basis for the virus plaque assay is the observation that normal lymphocytes are incapable of replicating RNA viruses while antigen- or mitogen-stimulated lymphocytes can do so. In addition to extending the assay from model system to more clinically relevant ones such as transplantation and tumor immunity, we hope to approach fundamental questions of the cell biology of the lymphocyte and virus-lymphocyte interactions to better understand the process of lymphocyte activation.